A gas turbine engine generally includes in serial flow communication, one or more compressors followed in turn by a combustor and high and low pressure turbines disposed about a longitudinal axis centerline within an annular outer casing. During operation, the compressors are driven by the respective turbines and compressor air which is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow downstream through the high and low pressure turbines which extract energy therefrom, for driving the compressors, and for producing other output power either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbo fan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers.
It is well understood that individual components in operation require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separator high and low turbines with independent power transmitting devices are necessary for the fan and core compressor in prior art aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power. These additional stages and the separate power transmitting devices result in weight penalties which are undesirable in aircraft applications.
Efforts have been made to minimize turbine weight of aircraft gas turbine engines by for example, a differential gearing system which distributes power from a single turbine to at least two different components, such as a core compressor and a fan rotor. This is known in the prior art, as described in U.S. Pat. No. 4,251,987, issued to Adamson on Feb. 24, 1981. In a differential geared turbine engine, the compressor, the fan and the turbine are all mechanically linked, and therefore modulating means are necessary to modulate the rotational speed and torque in order to optimize the individual component performances under various engine operation conditions. Various means are available within the existing technology for modulating the torque requirements of various components. Adamson suggests the use of known torque and flow varying techniques, such as variable pitch fans, variable core compressor stators, bleed air extraction, etc., which selectively vary the engine flow passage-defining geometry in order to modulate the torque versus speed characteristics of the individual components. However, a variable engine flow passage-defining geometry increases the complexity and therefore reduces the reliability of an aircraft gas turbine engine.
Use of machines operable as either generators or motors for shaft power transfer in gas turbine engines is known in the art. Hield et al. in their U.S. Pat. No. 5,694,765 which issued Dec. 9, 1997, describe a multi-spool gas turbine engine for an aircraft application, which includes a transmission system operated to transfer power between relatively rotatable engine spools. In a number of embodiments, each shaft is associated with a flow displacement machine operable as a pump or a motor, and in other embodiments, permanent magnet or electromagnetic induction type machines operable as motors or generators, are used. However, Hield et al.'s shaft power transfer system does not offer, disclose or teach differential geared gas turbine engines, because they direct themselves to the transfer shaft power between two independently rotatable (i.e. not differentially-geared) engine spools.
Therefore, it is desirable to provide an aircraft gas turbine engine configuration in which the turbine weight is minimized without compromising the engine flow passage-defining geometry thereof.